Inner ear disorders of hereditary origin are fairly common in humans. A full evaluation of these disorders cannot always, however, be studied in human material. Therefore, animals with hereditary inner ear defects are useful models for studying the basic mechanism in the complex picture of inner ear abnormality in humans. We uncovered a strain of new mutant mice which arose as a spontaneous mutation in the C3H/He stock. The animals show abnormal behavior such as circling, head-tossing and hyperactivity. The gene is autosomal recessive and a single gene mutation is expected on chromosome 11. The hearing ability and histological characteristics of the inner ear of this new mutant mice were analyzed. The audiological findings exhibited no recordable auditory brain stem response (ABR) in any homozygotes at ages ranging from 11 days to 117 days. The most striking morphological findings were disarray of the stereocilia of the outer hair cells of the cochlea and hair cells of the maculae, although hair cell cytoplasm became fully developed, including the nerve terminals. Age-dependent degeneration of the outer hair cells and saccular hair cells and disintegration of the saccular otoconia subsequently occurred. Because no morphological abnormality was observed in the central nervous system, the abnormal behavior in these mice was primarily correlated with morphological abnormalities of the vestibule. As this animal is expected to have a single gene abnormality, molecular genetic studies on this animal can provide important information on the nature of histological changes of the hair cell from mode of gene action.